| Radical reactions take an important role in the studies of the reaction mechanism and the reaction control. Radical reactions involve many fields, such as fields of combustion, explosion, environment and health. This thesis mainly investigated the reaction mechanism of ClO, OH and CCN radicals by using the quantum chemistry theory.In the first part of this thesis the potential energy surface of the CH2O + ClO reactionwas calculated at the QCISD(T)/6-311G(2d,2p)//B3LYP/6-311G(d,p) level of theory. The rate constants for the lower barrier reaction channels producing HOCl + HCO, H atom, OCH2OCl, cis-HC(O)OCl and trans-HC(O)OCl were calculated by TST and multichannel RRKM theory. Over the temperature range 200 - 2000 K, we predicted that the overall rate constants were k(200 - 2000 K) = 1.19×10-13T0.79 exp(- 3000.00/T) cm3 molecule-1s-1. At 250 K, the calculated overall rate constant was 5.80×10-17 cm3 molecule-1 s-1, which was in good agreement with the experimental upper limit. The calculated results demonstrated that the formation of HOCl + HCO was the dominant reaction channel and was exothermic by 9.7 kcal/mol with a barrier of 5.0 kcal/mol. When the products HOCl + HCO retrograded to the reactants CH2O + ClO, an energy barrier of 14.7 kcal/mol was required. Furthermore, when HOCl decomposed into H + ClO, the energy required was 93.3 kcal/mol. These results suggested that the decomposition in both the forward and backward directions for HOCl would be difficult in the ground electronic state. Thus, CH2O could be considered as a sink for ClO radicals.The second part involved theoretical studies on the reactions of OH radical with methyl sulfinic acid (CH3S(O)OH, MSIA), Peroxynitric Acid (HO2NO2, PNA) and Acetonitrile (CH3CN), respectively. The following results were obtained:(1) The potential energy surface and various reaction channels of the MSIA + OHreaction were investigated at the CCSD(T)/6-311+G(2d,p)//B3LYP/6-31+G(2d,p) level of theory. Four different hydrogen-bonded complexes were formed in the reactions. Theresults indicated that there were mainly two kinds of reaction mechanisms in the studied reactions, the direct CH3 radical abstraction reaction and the association-decomposition reaction. Owing to the higher barrier heights, the CH3-abstraction pathway was not expected to be the favorable pathway. While, the association-decomposition reaction pathway, MSIA-b + OH→M2b→TS9→H2O + CH3SO2→TS4 + H2O→SO2 + CH3 + H2O, was the most feasible reaction pathway, and SO2 was the dominant final sulfur-containing product.(2) The reaction of HO2NO2 (Peroxynitric Acid, PNA) with OH was studied by using the hybrid density functional B3LYP and CBS-QB3 methods. Based on the calculated potential energy surface, five reaction channels, H2O + NO2 + O2 (P1), HOOH + NO3 (P2), NO2 + HO3H (P3), HO2 + HONO2 (P4) and HO2 + HOONO (P5), were examined in detail. The major reaction channel was PNA + OH→M1→TS1→H2O + NO2 + O2. Taking the pre-equilibrium approximation, the theoretical rate constant of this channel was calculated by using the CBS-QB3 energies, which was 1.13×10-12cm3 molecule-1s-1 at 300 K and it was in agreement with the experimental result. Comparison between reactions of HOONO2 + OH and HONO2 + OH was carried out. For HOR + OH reactions, the overall rate constants increased from R = NO2 to R = ONO2, which was consistent with the experimental measurements.(3) The complex potential energy surface for the reaction of OH radical with CH3CN, including 2 intermediate complexes, 9 transition states, was theoretically probed at the CBS-QB3 level of theory. The geometries and relative energies for various stationary points were determined. Based on the calculated CBS-QB3 potential energy surface, thepossible reaction mechanism of OH + CH3CN was proposed. The rate constant (K1) for the lower barrier reaction channel producing P1 was calculated by TST theory and the plotof these rate constants over the temperature range 250 - 3000 K was also obtained. Theresults indicated that K1 was increasing with the temperature in the range 250 - 3000 K. By comparison with the obtained experimental values we concluded that P1 was thedominant product in the temperature range 250 - 3000 K. The results showed that the values of K1 were in good agreement with the experimental results over the temperature range 250 - 320 K.In the third part of this thesis the potential energy surface and the various reaction channels of the CCN + H2X (X = O, S) reaction were investigated at the CCSD(T)/6-311+G(2d,p)//B3LYP/6-31+G(2d,p) level of theory. For the CCN radical the terminal C atom has a vacant orbital and a lone pair of electrons, and it was expected to bemuch more reactive than the N atom. The results indicated that the mechanism of the carbon insertion-dissociation was more favorable than that of the direct H abstraction inthe reaction system of CCN + H2X (X = O, S). The calculation results showed that H + OCHCN was the main product in the reaction of CCN + H2O, which could be obtained through two different pathways. On the contrary, the main product was H + SCHCN in the reaction of CCN + H2S, and it could be formed by only one reaction channel.
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